Measuring equipment for use in connection with hip prosthesis surgery

ABSTRACT

The invention concerns a measuring instrument, preferably for use in connection with press fit hip prosthesis surgery, whereby it is possible to measure/register size (diameter) and modulus of elasticity of acetabulum and femur for inserting an uncemented socket or uncemented femur component.

SCOPE OF THE INVENTION

The present invention concerns a measuring instrument, preferably foruse in connection with hip prosthesis surgery.

BACKGROUND OF THE INVENTION

Through the last 35-40 years bone cement has been used for anchoringprosthesis components when inserting artificial hip joints. Good resultshave been achieved with this technique, but after long time observationof large groups of patients, problems have been detected with looseningof the inserted prosthesis component. Furthermore, a considerable boneloss has occurred around the prosthesis component so that a replacementsurgery is made difficult.

Through the years, alternative techniques in the shape of uncementedsockets, where the socket is anchored in acetabulum, have beendeveloped. Different anchoring methods have been developed, and themethod appearing to provide good results on short as well as long viewis the use of metal sockets, preferably made of materials as e.g.titanium or vitalium. These metal sockets are typically hemisphericalwith a porous surface.

The surface of the metal socket is applied a so-called porous coatingimplying that the surface is rough and provides possibility of ingrowthof bone cells into the surface of the metal socket. The surface coatingof a metal socket typically has a pore size of 50-250 μm and may eitherconsist of small metal balls or metal fibres that are compressed orsprayed on. A supplementing hydroxapatite coating promotes the boneingrowth process.

When inserting an uncemented porously coated metal socket, a holefitting the metal socket to be inserted is milled in acetabulum. E.g. ifacetabulum is milled up to 60 mm, a 60 mm metal socket is inserted. Thenthe metal socket is anchored further with 2-4 screws through holes inthe metal socket. However, screw anchoring has appeared to haveunfavourable effects, as the polyethylene decomposition product canmigrate through the screw holes and out into the bone, and here produceosteolysis or necrosis of the bone.

In order to avoid the latter, in later years another anchoring methodhas been used, consisting in that a so-called “press-fit” technique hasbeen used, where e.g. acetabulum is milled up to 60 mm in diameter,where subsequently is inserted e.g. a 62 mm or 64 mm metal socket. Agreat tension is thereby achieved between bone and metal socket, wherebythe metal socket is held in place without supplementing use of screws.In the course of the first weeks after the operation, the bone thengrows from the pelvis into surface coating of the metal socket andensures the so-called secondary anchoring.

The described technique is, however, rather demanding and requires anexperienced physician to perform the hip prosthesis surgery, as there isrisk of problems if the metal socket has not been correctly inserted. Ifthe metal socket is not securely clamped, there is risk that it mayloosen in the course of the first days after the operation. If the metalsocket is too strongly clamped, there is great risk of fracture in thepelvis bone around the metal socket whereby it becomes unstable.

Therefore, it is important to insert the right size of metal socket. Inorder to determine which size of metal socket to be used, previously atemplate or a test prosthesis has been used and placed in acetabulum andwhich by a light pressure is fixed in acetabulum, after which isselected a metal socket in the same size or with 1-2 mm oversize.

However, it is not a good and objective method of measuring, and one mayrisk to use/insert more than one metal socket in order to let it fitoptimally. A great disadvantage by this method is that a metal socketthat has been in contact with a patient cannot be autoclaved and usedfor another patient. This metal socket is to be discarded, which alsoimplies increased costs in connection with a hip prosthesis surgery.

In patent publications FR 2 684 287 and U.S. Pat. No. 5,141,512 there isdescribed equipment for measuring in connection with placing of a hipsocket, but not decidedly measuring the size and the elasticity ofacetabulum.

Until now, no equipment has been developed with which the physician canmeasure the hip socket and thereby objectivise this part of theprocedure of hip surgery. E.g. the risk of fracture in the hip socketdepends on the elasticity in the bone and the size of the acetabulum.Furthermore, it also has great significance whether it is a first timeoperation or a replacement operation.

OBJECT OF THE INVENTION

It is the purpose of the present invention to indicate a measuringinstrument which in a simple way can measure/register the size and theelasticity as well as the shape of acetabulum. This implies that hipprosthesis operations become safer, and the costs of these operationsare reduced.

This is achieved according to the present invention with a measuringinstrument of the kind mentioned in the introduction, which is peculiarin that the measuring instrument includes:

-   -   an adapter including a measuring head and a connector, where the        measuring head is provided with a largely hemispherical surface        in which there is a central through-going bore, where the        measuring head is divided into at least two separate sections,        where the connector is hollow and slotted at one end in axial        direction of the measuring head into a number of legs, the        number of legs corresponding to the number of the at least two        separate sections of the measuring head, where the legs of the        connector are each connected to one of the at least two sections        of the measuring head on the rear side of the hemisphere, where        a securing arrangement is provided at the opposite end of the        connector relative to the connection to the measuring head;    -   a measuring unit including a actuating rod, where a main part        having preferably conical shape that can interact displaceably        axially with the central bore of the adapter and thereby change        the diameter of the measuring head of the adapter, a mounting        ring where means for engaging the securing arrangement of the        adapter are provided, where the mounting ring is connected with        a handle member, where in connection with the handle member        there is provided means for axial displacement of the actuation        rod and registration of the relative displacement of the        actuation rod relative to the measuring head;    -   a depth gauge including a first part, which is preferably a        smooth rod, and a second part with stop and measurement        indications, where the depth gauge is provided for interacting        with a through-going opening in the axial direction of the        measuring instrument.

DESCRIPTION OF THE INVENTION

A measuring instrument according to the invention will make it easy andsimple to measure the diameter of an acetabulum by a hip operation.

Before inserting a metal socket in acetabulum, the measuring instrumentis used in that:

-   -   an adapter with a measuring head with a desired diameter is        coupled to the measuring unit by means of the coupling        arrangement and the mounting ring,    -   the measuring head of the adapter is inserted into the        acetabulum so that the lower edge of the hemispherical measuring        head fits with the circumferential rim of the acetabulum,    -   by actuating the handle means for axial displacing the actuation        rod, the actuation rod will perform an axial displacement        towards the measuring head,    -   by the axial displacement, the main part of the actuation rod        will interact with the preferably conical opening inside the        measuring head and thereby force the separate sections of the        measuring head outwards,    -   spreading of the separate sections of the measuring head cause        the diameter of the hemispherical measuring head to expand,        whereby the measuring head is clamped in the acetabulum,    -   at a certain pre-tensioning, the diameter of the measuring head        is read,    -   the depth gauge is used for measuring the possible distance from        the surface of the measuring head and down into the bottom of        the acetabulum,    -   after measuring diameter and depth of the acetabulum, the handle        means are actuated for axial displacement of the actuation rod        so that the actuation rod performs an axial displacement in        against the handle, and the measuring head may be removed from        the acetabulum.

In order to place the measuring head of the adapter in the acetabulumand being sure that the right diameter of the acetabulum is measured,the spherical surface of the adapter measuring head is provided with atleast two different surfaces, a rough surface on the lower part of thespherical surface, and a smooth surface on top of the spherical surface.

The rough surface provides for better engagement of the sphericalsurface of the grooved/milled bone tissue of the acetabulum. By usingthe measuring instrument, the measuring head is placed against the edgeof the acetabulum, whereby the rough surface prevents displacing of themeasuring head during expansion of the measuring head. The rough surfacemay be provided with different patterns which can be an arbitrary numberof strings, linear and/or crossing flutes and/or beads.

Since only a few acetabuli have largely hemispherical shape, it is notnecessary that the entire spherical surface of the measuring head has arough surface. Therefore a part of the spherical surface of themeasuring head is provided with a smooth surface, since a smooth surfacewill be easier to keep clean and thereby more hygienic.

In a preferred embodiment of the invention, the adapter is connectedwith the measuring unit in that the securing arrangement of the adapterare recesses for a bayonet socket, and that the mounting ring of themeasuring unit, where means for engaging the securing arrangement of theadapter are provided, are designed as pins for a bayonet engagement.

By using a bayonet engagement it is possible to readily assemble theadapter with the measuring unit. Furthermore, a bayonet engagement isnot a difficult assembly to manufacture, irrespectively whether theadapter is cast in one or the connector of the adapter is a pipe.

Alternative embodiments for the securing arrangement between the adapterand the measuring unit may e.g. be:

-   -   a screw thread providing a very stable assembly, but having the        drawback that it may be difficult to clean;    -   a snap lock performing a releasable engagement when the adapter        is placed in the measuring unit, implying easy and quick        replacement of an adapter, but having the drawback that they may        be worn out and thereby cannot hold the adapter in the measuring        unit.

If the measuring instrument is made of a sufficiently cheap material,the adapter and the measuring unit may be made in one piece. Thisimplies that for each hip operation, a series of measuring instrumentswith different diameter of the measuring head are to be used. Followinguse, the used measuring instruments are to be discarded or autoclaved.

For the measuring head of the adapter to expand while at the same timethe adapter being easy and cheap to manufacture, the adapter is to bedesigned so that the adapter is made of a hard yet flexible materialwhich e.g. may be one or more of the following: a metal alloy, a plasticmaterial, e.g. a plastic composite, and/or ceramic material.

It will be an advantage to avoid joints in the adapter and, if theadapter is made of a metal alloy, that the adapter is made in one piecewhere the adapter either is cast with cutouts and recesses, or that theyare made subsequently. Furthermore, the metal is also to have a certaintemperature resistance as an adapter made in a metal alloy will be anexpensive equipment and is therefore to be autoclaved after use.

In a preferred embodiment of the invention, the adapter is made of ahard yet flexible plastic composite, e.g. PDM or the like. By making theadapter of a plastic composite, the adapter may either be moulded in thesame way as when making an adapter of a metal alloy, or it may berotationally moulded, where the subsequent processing of making of slotsand recesses may readily be performed afterwards.

In an alternative embodiment of the invention, the adapter may be madein a ceramic material where a health effect may be achieved, sincemetals or plastics are not to be introduced in an open operation region.When using ceramics, a suitable manufacturing method is to be found sothat the measuring head of the adapter may expand. A suitablemanufacturing method may, however, increase the cost of making adapters.

However, it is important that all materials are heat resistant, so thatirrespectively of the material the adapter and the measuring unit aremade of, it is to be possible to autoclave the entire measuringinstrument or parts of the measuring instrument.

In order that the measuring head of the adapter may expand by an axialmovement, the preferably conical main part of actuation rod willinteract with a face and/or edge of the central through-going boreinside the measuring head of the adapter.

In neutral position of the actuation rod the at least two separatesections of the measuring head are largely gathered, and by axialdisplacing of the actuation rod, the at least two separate sections ofthe measuring head will be pressed away from each other, and thediameter of the measuring head will expand with Up to 4 mm. A greaterexpansion of the diameter of the measuring head than 4 mm may causefracture in acetabulum.

The central through-going bore in the axial direction enables passingthe actuation rod through the connector, up inside and out through themeasuring head. The bore in the measuring head is designed so that it ispreferably conical.

The main part of the actuation rod, the shape of which is conical, willby axial displacement of the main part of the actuation rod into thepreferably conical bore in the measuring head cause the three sectionsof the main part to be pressed away from each other.

In an alternative embodiment of the through-going bore in the measuringhead and/or the main part of the actuation rod, it is e.g. possiblethat:

-   -   the through-going bore in the measuring head is designed as a        straight bore and the main part of the actuation rod is designed        largely conical, or ball-shaped, whereby the main part of the        actuation rod interacts with a rim part of the through-going        bore in the measuring head;    -   the through-going bore in the measuring head is designed largely        conical, or with curved faces, and the main part of the        actuation rod is designed as a rod with increased diameter in        relation to the actuation rod itself, whereby the upper edge        section of the main part of the actuation rod interacts with        surfaces of the through-going bore in the measuring head;    -   the through-going bore in the measuring head is designed largely        conically or with curved faces, and the main part of the        actuation rod is designed largely conical or spherical, whereby        surfaces of the main part of the actuation rod interacts with        surfaces of the through-going bore in the measuring head.

In the two first alternatives, it will be necessary to safeguard thefaces of either the through-going bore in the measuring head or the mainpart of the actuation rod, as they are to interact with an edge sectionof the opposing part. By repeated actuation of the actuation rod, wearmay occur where the edge section hits the opposing surface. A safetymeasure may be surface coating of the exposed surfaces, or by making themeasuring head and the main part of the actuation rod of materials withdifferent hardness, so that the item with the edge part is made of thesoftest material.

The adapter is designed with a measuring head which is largelyhemispherical and divided into at least three sections. This division ismade in order to achieve a satisfactory contact of the surface of thespherical face against the acetabulum, particularly by expansion of themeasuring head.

These sections are connected to the legs of the connector which areprovided by one end of the connector being slotted in axial direction.The slotting provides that the connector can absorb the forces comingfrom the ball head when it expands.

In a preferred embodiment of the invention, means of axial displacementof the actuation rod is a millimetre screw device. Such a millimetrescrew device will enable placing the measuring head of the adapter in anacetabulum, make it fit and thus, without further actuation in axialdirection, enable screwing on the millimetre screw device so that theactuation rod is displaced axially into the measuring head bore, andthereby increasing the diameter of the measuring head.

Such a millimetre screw device includes a spring in connection with anadjustment part and a swivel ring with one or more measuring indicators.Furthermore, it may also be a ratchet function inside the millimetrescrew device.

In an embodiment of the invention, the millimetre screw device is madeso that it may “click over” (overload protection) in order to avoid toomuch expansioning of the measuring head, and too strong clamping of themeasuring head in the acetabulum, which may lead to fracture of theacetabulum.

The measuring indicators on the measuring ring and on the measuring unitprovide that by counting turns of the swivel ring and by readingmeasuring units by means of conversion factor, the increase of thediameter of the measuring head may be determined.

The adjusting part is used every time a new adapter is inserted in themeasuring instrument, whereby the measuring instrument is adjusted toneutral position.

In an alternative embodiment of the invention, the means for axialdisplacement of the actuation rod has been selected among one or more ofthe following: hydraulics, pneumatics and/or electricity driving a motoror a unit that interact with the actuation rod.

In order to enable fine adjustment of the diameter of the measuring headin the acetabulum, it may be an advantage that axial displacement of theactuation rod is motorised. Besides that it will not be the the doctor'sability of setting the measuring instrument which is decisive for thesocket size, at the same time problems with holding the measuring headin the socket simultaneously with screwing or moving the actuation rodmanually in axial displacement are avoided. This will make the measuringinstrument lighter and the measurements more objective in use.

For measuring the actual displacement of the actuation rod, registrationof the relative displacement of the actuation rod in relation to themeasuring head occurs by reading one or more measuring indicationsapplied on the handle.

These measuring indicators are to be applied clearly so that a user ofthe measuring instrument does not confuse the measurements read, whichmay cause a wrong metal socket to be pressed into the acetabulum,whereby problems like bad fixation of the metal socket or fracture inacetabulum may arise.

In an embodiment of the invention, the measuring instrument is connectedto a computer for controlling and/or registering the relativedisplacement of the actuation rod relative to the measuring head.

In a further embodiment of the invention, the measuring instrument maybe used for collecting data on a computer connected thereto. Thiscomputer may either be coupled to the motor/unit and thereby measure theturns that correspond to a certain relative displacement, or be coupledto a measuring arrangement which is capable of measuring, e.g. by meansof optics, lasers or the like, how long the relative displacement of theactuation rod is in relation to a fixed point in the measuringinstrument.

By collecting data so that one may have a large database in thecomputer, whereby more and more experience is collected as to how anacetabulum is shaped concurrently with more and more data beingregistered with the measuring instrument. This makes it easier andquicker to find the socket fitting optimally with acetabulum.

Furthermore, such a system with a computer for controlling andregistering will be a good tool in teaching inexperienced/new physiciansfor hip/prosthesis operations as experiences from previous operationsare stored in the database. This makes it possible for the doctorsperforming hip prosthesis surgery to draw on experiences/informationbefore they commence the operation.

In order to optimise the hip prosthesis operation, it is important forthe doctors to measure the pressure to which the acetabulum is subjectedat the insertion of the metal socket, as the pressure most likely willbe different with regard to different groups of patients. E.g. it may bedifficult to achieve sufficient stability of the metal socket inpatients with osteopsathyrosis.

The pressure exerted by the metal socket on acetabulum may be measuredsimultaneously with measuring the diameter of acetabulum with ameasuring instrument according to this invention. The measuringinstrument is provided with a pressure registering unit connected to themeasuring head, so that simultaneously with registration of the increaseof the diameter the pressure action of the measuring head on acetabulumis measured.

The pressure registration unit may e.g. be pressure transducers that areprovided in or on the measuring head. They are, however, to be durableand securely fastened as the measuring head is forced around inacetabulum in order to ensure selection of the correct metal socket.

Alternatively, piezo-electric crystals may be used for pressureregistration on the measuring head.

By comparing pressure measurement, diameter of the metal socket and thepatient's data, the doctors may reduce the problems with loose metalsockets and fracture of acetabulum implying that the patient isimmediately to undergo a new operation or is to have a replacementoperation earlier than expected.

In order to have a flexible system for measuring acetabulum during hipoperation, the adapter for use in the measuring instrument isreplaceably connected with the measuring unit of the measuringinstrument. This provides for the adapter and the measuring unit beingmade of different materials.

An adapter will typically be made of a more cheap material than themeasuring unit, whereby it is possible to practice a method where anadapter is used once and then discarded. This results in saving ofautoclaving of an adapter after use, as well as the risk of infection orother unwanted contamination is reduced.

Furthermore, by using a replaceable adapter it is possible to provide aseries of adapters with different diameter of the measuring head, wherethe measuring head diameter in neutral position may vary from 45 to 85mm, preferably between 50 and 70 mm.

It is advantageous to provide an adapter series where the measuring headhas different diameter, as the measuring instrument may be used for alltypes of hip operations by replacing the adapter. This means that ameasuring instrument with adapter with a small measuring head may beused for persons by pelvis operations where the acetabulum does not haveso large diameter, whereas an adapter with large diameter is preferablyused in pelvis operations where the acetabulum has large diameter.

A series of adapters with different diameters of the measuring head isprovided with diameters in steps of about 2 mm measured when themeasuring head of the adapter is in neutral position and is notinfluenced by the actuation rod. However, the step size of the measuringhead diameter of the adapter is to be maximum 3 mm if the diameter ofthe adapter measuring head can expand up to 4 mm.

This implies that the adapter is not to be brought to an extremeposition of its measuring range before it is required/possible toexchange the adapter with an adapter with greater measuring headdiameter. Thereby, for an inexperienced and/or unskilled physician thereis achieved a rapid and more precise measurement of the diameter of theacetabulum.

Furthermore, it implies that it is easier to find an adapter thatlargely fits into the acetabulum, after which the actual diameter of theacetabulum is produced by fine adjustment of the measuring unit.

For adapters made of a material by which it is economically advantageousto reuse the adapters after autoclaving, it is a further advantage thatby repeated use of an adapter there is not exerted an unnecessary stresson the legs on the connector of the adapter due to large expansions ofthe measuring head. The adapter may hereby be used more times and lastlonger.

An adapter that fits within about 2 mm when inserted in an acetabulumwill facilitate the work in measuring the diameter of the acetabulum asthe doctor is not simultaneously to hold the measuring head of theadapter in position in the acetabulum so that the rough surface of themeasuring head engages the edge of the acetabulum and to screw/turn ordisplace the actuation rod.

When the diameter of the metal socket to be inserted in the acetabulumhas been found, the measuring instrument is designed so that a depthgauge interacts with a through-going opening in the axial direction ofthe measuring instrument.

In the preferred embodiment, the depth gauge will be provided through acentral through-going opening in the axial direction of the measuringinstrument so that it goes through the measuring unit, out through theactuation rod and out through the through-going bore of the adapter.This implies that the depth gauge by mounting through the measuring unitwill project at the point of the ball-shaped measuring head of theadapter.

The depth gauge is used for measuring the depth of acetabulum so that itmay be determined how much bone implant (artificial bone mass and/orbone mass taken from the patient) which is to be disposed betweenacetabulum and metal socket in order that the metal socket is provided anatural and solid bed in acetabulum.

On the depth gauge is indicated a measuring indication so that inneutral position a first measuring mark is indicated, after whichmeasuring markings with suitable spacing are indicated. This spacingbetween the measuring indications will typically indicate one or twomillimetre at a time.

The principles described in the present invention may also findapplication in connection with insertion of uncemented prosthesiscomponents in other human joints, e.g. in the marrow cavity of the femuror in the neck of the femur.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will then be explained more closely with reference to theaccompanying drawing, where:

FIG. 1 shows a measuring instrument according to the invention,

FIG. 2 shows a side view of an adapter according to the invention,

FIG. 3 shows a cross-section of the measuring head of the adapter,

FIG. 4 shows a cross-section of the interaction between measuring headand an actuation rod,

FIG. 5 shows a plan view of the measuring head,

FIG. 6 shows the connector of the adapter,

FIG. 7 shows the depth gauge of the measuring instrument,

FIG. 8 shows a cross-section of a metal socket inserted in acetabulum,and

FIG. 9 shows a graph of the relation between the force with which themeasuring head acts on acetabulum and the diameter of the measuringhead.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is shown a measuring instrument 1 including an adapter 2 and ameasuring unit 3.

The adapter 2 includes a connector 4 and a measuring head 14 that has ahemispherical surface 5. The connector 4 has at least two axiallyextending slots 6 dividing the end 15 of the connector 4 into a numberof legs 7. The measuring head 14 includes a hemispherical surface Sdivided into a number of sections (not shown) so that each part isconnected with one leg 7 of the connector 4. At the end 16 of theconnector 4 are provided recesses 9 that are designed to interact withcorresponding pins (not shown) provided in the mounting ring 12 of themeasuring unit 2. Inside the measuring unit 14 is provided an axiallythrough-going conical bore 8 which is designed to interact with theactuation rod 10 of the measuring unit 3.

The measuring unit 3 includes an actuation rod 10, a mounting ring 12and a millimetre screw device 13. The actuation rod 10 has a conicalmain part 11 which by axial displacement and engagement with the conicalbore 8 in the measuring head 14 forces the separate sections (not shown)of the measuring head 14 from each other. The outward going force causedby the displacement of the actuation rod 10 in relation to the conicalopening 8 of the measuring head is absorbed in bending the legs 7 of theconnector 4.

The mounting ring 12 is provided with pins (not shown) which is designedto engage the recesses 9 in the end 16 of the adapter 2.

In order to achieve axial displacement of the actuation rod 10 inrelation to the conical opening 8 of the measuring head 14, themeasuring unit 3 is provided as a millimetre screw device 13 which byrotation axially displaces the actuation rod 10. Inside the millimetrescrew device 13 is provided a releasable ratchet device 17 that ensurestightening of the millimetre screw device 13.

FIG. 2 shows the adapter 2 which includes a connector 4 and a measuringhead 14 having a hemispherical surface 5. In the connector 4 there isshown axially extending slots 6 that divide the end 15 of the connector4 into a number of legs 7. The slot 6 ends in a circular opening 19 thatprevents notch effect in the connector 4 when the measuring head 14expands.

The measuring head 14 includes a hemispherical surface 5 which isdivided into a number of sections 30 each connected with one of the legs7 of the connector 4. The hemispherical surface 5 of the measuring head14 is divided into two areas with different roughness: a top part 31with smooth surface and a bottom part 18 with rough surface.

FIG. 3 shows a cross-section in the measuring head 14. Inside themeasuring head 14 is provided an axial through-going conical bore 8which is designed to interact with actuation rod (not shown). In orderto bear against and produce some resistance against acetabulum, thesurface 5 of the measuring head 14 is divided into a top part 31 with asmooth surface and a bottom part 18 with a rough surface.

FIG. 4 shows a cross-section of the measuring head 14 by actuation ofthe actuation rod 10, where the conical main part 11 of the actuationrod 10 is engaging the conical opening 8 of the measuring head 14. Anupwards directed axial displacement of the actuation rod 10 will resultin the sides 32 on the conical main part 11 of the actuation rod 10forcing the sections 30 of the measuring head from each other, and theslot 33 will be enlarged.

FIG. 5 shows a plan view of a measuring head 14 as seen from the bottomand up through the conical opening 8. The three sections 30 of themeasuring head are separated by the slots 33.

FIG. 6 shows the connector 4 of the adapter 2 in which is provided aslot 6 opening into a circular opening 19 that divides the connector 4in the legs 7. Furthermore is shown a recess 9 at the end 16 of theconnector 4 which is designed to interact with corresponding pins (notshown) provided in the (not shown) mounting ring of the measuring unit.

FIG. 7 shows the depth gauge 35 of the measuring instrument 1 thattogether with the measuring instrument 1, where a through-going openingcorresponding with the diameter 35 of the depth gauge has been providedinside through all the components of the measuring instrument 1. Thedepth gauge preferably includes a long, slender body part 36 terminatingin an end part 37, which by use of the depth gauge 35 during use of themeasuring instrument (not shown) reaches the bottom of the reamedacetabulum (not shown). At the opposite end, there is a handle part 38enabling the doctor to operate the depth gauge 35. At the side of thehandle 38 there are provided a number of measuring indications 39indicating the depth (volume) of the cavity between a metal socket (notshown) of a given diameter and acetabulum.

FIG. 8 shows a metal socket 20 inserted in an acetabulum 21. The metalsocket 20 is inserted according to the press-fit technique and isoversize compared with acetabulum 21. Therefore, the outwards directedforces 23 will arise, and the metal socket 20 will be securely anchoredin the acetabulum. Behind the metal socket 20 an interspace 24 typicallyappears which is filled with bone mass so that the metal socket 20 issecured in a natural and solid bed in acetabulum 21. It is the depth ofthis interspace 24 which is measured with the depth gauge (not shown).

It is the outwards directed forces 23 that are very necessary in thelong nm for measuring in order to optimise hip prosthesis operations.FIG. 9 shows a graph over the outwards directed forces 23 and thediameter of the measuring head 14. The graph 40 shows the correlationbetween the force F [Nm] with which the metal sockets acts onacetabulum, particularly the sides in the inlet hole.

The example illustrated in FIG. 9 is for a patient having reamed a 64 mmhole in acetabulum. By inserting a measuring head of 64 and subsequentlyexpanding the measuring head, the force F will be increased until themeasuring head has expanded so much that it causes fracture. Thefracture is illustrated by the graph 40 dropping drastically when thediameter goes up to 68 mm or more.

As such a fracture is not only dependent on the size of the insertedmeasuring head/metal socket, but also on the bone condition of thepatient, it will be a great advantage if the force F is measured whileusing the measuring instrument.

The invention is not limited to the embodiments shown in the Figures anddescribed above. Other embodiments including other kinds of couplingarrangements between adapter and measuring head, surface and internalshapes of the measuring head of the adapter and methods of driving theactuation rod are conceivable within the frames of this invention andthe features specified in the claims.

1. Measuring instrument, preferably for use in connection with hipprosthesis surgery, characterised in that the measuring instrumentincludes: an adapter including a measuring head and a connector, wherethe measuring head is provided with a largely hemispherical surface inwhich there is a central through-going bore, where the measuring head isdivided into at least two separate sections, where the connector ishollow and slotted at one end in axial direction of the measuring headinto a number of legs, the number of legs corresponding to the number ofthe at least two separate sections of the measuring head, where the legsof the connector are each connected to one of the at least two sectionsof the measuring head on the rear side of the hemisphere, where asecuring arrangement is provided at the opposite end of the connectorrelative to the connection to the measuring head; a measuring unitincluding a actuating rod, where a main part having preferably conicalshape that can interact displaceably axially with the central bore ofthe adapter and thereby change the diameter of the measuring head of theadapter, a mounting ring where means for engaging the securingarrangement of the adapter are provided, where the mounting ring isconnected with a handle member, where in connection with the handlemember there is provided means for axial displacement of the actuationrod and registration of the relative displacement of the actuation rodrelative to the measuring head; a depth gauge including a first part,which is preferably a smooth rod, and a second part with stop andmeasurement indications, where the depth gauge is provided forinteracting with a through-going opening in the axial direction of themeasuring instrument.
 2. Measuring instrument according to claim 1,characterised in that spherical surface of the measuring head of theadapter is provided with at least two different surfaces, a roughsurface at the lower part of the spherical surface and a smooth surfaceon top of the spherical surface.
 3. Measuring instrument according toclaim 1, characterised in that the securing arrangement of the adapterare recesses for a bayonet socket.
 4. Measuring instrument according toclaim 1, characterised in that adapter is made of a hard yet flexiblematerial, which can be one or more of the following: a metal alloy, aplastic material, e.g. a plastic composite, and/or ceramic material. 5.Measuring instrument according to claim 1, characterised in that thepreferably conical main part of the actuation rod interacts with a faceand/or edge of the central through-going bore inside the measuring headof the adapter, where in neutral position of the actuation rod the atleast two separate sections of the measuring head are largely gathered,and by axial displacing of the actuation rod, the at least two separatesections of the measuring head will be pressed away from each other, andthe diameter of the measuring head will expand with up to 4 mm. 6.Measuring instrument according to claim 1, characterised in thatmounting ring of the measuring unit, where means for engaging thesecuring arrangement of the adapter have been provided, is designed aspins for a bayonet socket.
 7. Measuring instrument according to claim 1,characterised in that means for axial displacement of the actuation rodis a millimetre screw device.
 8. Measuring instrument according to claim7, characterised in that the millimetre screw device includes a springin connection with an adjusting member and a swivel ring with one ormore measuring indications.
 9. Measuring instrument according to claim1, characterised in that means for axial displacement of the actuationrod is one or more of the following: hydraulics, pneumatics and/orelectricity driving a motor or unit interacting with the actuation rod.10. Measuring instrument according to claim 1, characterised in thatregistration of the relative displacement of the actuation rod relativeto the measuring head is effected by reading one or more measuringindications on the measuring handle.
 11. Measuring instrument accordingto claim 1, characterised in that the measuring instrument is connectedto a computer for controlling and/or registering the relativedisplacement of the actuation rod in relation to the measuring head ofthe relative displacement of the actuation rod.
 12. Adapter for use in ameasuring instrument according to claim 1, characterised in that theadapter is exchangeably connected with the measuring unit of themeasuring instrument.
 13. Adapter according to claim 10, characterisedin that the adapter is provided in a series with different diameters ofthe measuring head, where the diameter of the measuring head in aneutral position can vary from 45 to 85 mm, preferably between 50 mm and70 mm.